Electron beam writing techniques have been satisfactorily employed as a means of pattern writing chips on a semiconductor wafer. The technique requires the writing to be achieved under very high vacuum conditions such as 3.times.10.sup.-6 torr, for example. The semi-conductor wafer is required to be introduced to vacuum condition from a position exterior of the vacuum chamber which is essentially at atmospheric pressure, and removed from the vacuum chamber thereto, after pattern writing.
An apparatus for handling work pieces satisfactorily and for facilitating such wafer movement to and from the vacuum chamber, where electron beam application occurs, is set forth in U.S. Pat. No. 3,968,885 issuing July 13, 1976, entitled "Method and Apparatus for Handling Work Pieces" by Javathuk Hassan et al, and assigned to the common assignee. The transport of the semiconductor wafers therein, is achieved at relatively high speed by the utilization of a virtual antechamber formed by movable elements within and exterior of the main vacuum chamber itself. By employing an antechamber having a relatively small volume in comparison with that of the vacuum chamber, and on the order of approximately one hundredth the size of the vacuum chamber, the necessity to have the same vacuum level existing within the antechamber as the vacuum chamber is eliminated, since the pressure conditions in the relatively small antechamber are not sufficiently significant to affect the vacuum level within the vacuum chamber when communication is established between the antechamber and the vacuum chamber. However, due to the relatively small volume and lesser vacuum level required of the antechamber, the time for pumping down the antechamber, after presentation of a semi-conductor wafer to that antechamber at atmospheric pressure, is considerably decreased over prior apparatus requiring the bringing of an auxiliary chamber to essentially the same vacuum pressure conditions as the vacuum chamber in which electron beam pattern writing of the semiconductor wafer is effected. Additionally, the elevator functions as a valve between the antechamber and the vacuum chamber, closing off communication therebetween in addition to raising and lowering the semiconductor wafer to and from a position within the antechamber where it is accessible to the exterior of the vacuum chamber. Once within the vacuum chamber, the wafer is sealed from the exterior, permitting lateral transfer from the elevator to an X-Y table, to one side thereof, where the wafer is precisely positioned with respect to an impinging electron beam.
In order to accomplish the raising and lowering of the elevator bearing the wafer between a first, raised or upper position in which the wafer is presented to the antechamber and sealed from the vacuum chamber itself, at which point, the cover or lid of the antechamber may be raised to provide access from the exterior of the vacuum chamber to the elevator for either placement of a wafer on the elevator or removal therefrom, and a second lowered position with the wafer positioned within the vacuum chamber and accessible to a lateral transfer mechanism for movement of the X-Y table, the apparatus of the above identified patent utilizes a drive mechanism comprising a tandem air/oil cylinder. The air cylinder functions as the driving force and the oil cylinder functions as a regulating device. However, the drive cylinder is sized to overcome the force generated when the top of the elevator, which seals itself to the top plate of the apparatus defining the vacuum chamber and which is open to the antechamber, is exposed to the atmosphere, upon raising of the antechamber lid or cover. With the vacuum chamber at very high vacuum pressure and the top of the elevator at atmospheric pressure, a considerably large pneumatic and/or hydraulic force is required to drive the cylinder against an applied pressure differential which may be on the order of 1000 pounds. Further, if the elevator and the internal transfer mechanism transferring the wafers laterally to and from the elevator is operated out of sequence, major damage is caused to these components and to the wafer carrier borne by the elevator. This may result in several days of shut down time to repair the apparatus.
It may be appreciated that the elevator drive must operate under certain different air pressure/vacuum pressure conditions;
(1) a condition, in an exemplary apparatus wherein the antechamber and the main chamber are both at high vacuum and in which there is approximately 140 pounds of force acting in the upward direction on the drive mechanism caused by atmospheric pressure acting on the bottom of the elevator which projects sealably through the bottom wall of the main vacuum chamber; PA1 (2) a condition where the elevator is in its fully raised or up position, sealed to the top wall of the main vacuum chamber and sealing off the antechamber to the vacuum chamber. With the vacuum chamber at high vacuum pressure and the antechamber at atmospheric pressure, the difference between the force caused by the atmosphere acting on the bottom and top of the elevator and the weight of the mechanism causes, for example, a force of approximately 990 pounds to act on the drive mechanism in the downward direction. (This is the condition discussed previously); PA1 (3) a condition in which the complete system is at atmospheric pressure and the weight of the elevator components applies a force of 50 pounds in the downward direction on the drive mechanism.
It is therefore an object of the present invention to provide an improved load elevator drive mechanism in which the loads encountered by pressure differential acting on the elevator are counterbalanced and dampened during varying antechamber and vacuum chamber pressure conditions, thereby allowing the elevator to be driven up and down with minimal force, and to allow the pressure differential to self seal the elevator to the vacuum chamber wall upon termination of elevator up drive and removal of the counterbalance force.
It is a further object of the present invention to provide an improved load elevator drive mechanism for an electron beam semiconductor wafer writing apparatus which eliminates shock and vibrating movement to the electron beam column to thereby permit electron beam writing on one wafer simultaneous with loading and/or unloading of a second wafer to and from the vacuum chamber where writing is occurring.
It is a further object of the present invention to provide an improved load elevator drive mechanism for an electron beam writing apparatus which includes positive mechanical locking means for preventing lowering of the elevator when the elevator top is exposed to atmospheric pressure and the vacuum chamber is at high vacuum.